One way to prevent or reduce the severity of social and linguistic communication disorders in children at high risk for these disorders (e.g., younger siblings of children with autism spectrum disorders, Sibs-ASD) is to treat them at an age when the brain is still developing (i.e., infancy). Current screening methods over-identify which Sibs-ASD need expensive treatments. An Incremental Treatment Approach (1) uses a cost-effective treatment with all risk group members, (2) identifies those who do not change on important skills after cost- effective treatment (i.e., nonresponders), and (3) provides nonresponders with a more intensive (and expensive) treatment. This project will address the first two steps of the Incremental Treatment Approach. Implementing the first two steps may avoid costly treatment of a subgroup of at-risk infants who may improve without intensive and expensive intervention. One type of cost-effective treatment that may allow us to accomplish the 2nd step targets specific child abilities that promote communicative development (i.e., pivotal skills). Furthermore, identifying the characteristics that distinguish Sibs-ASD most likely to benefit from the cost-effective treatment from Sibs-ASD who will require more intensive intervention will promote effective and efficient implementation of the Incremental Treatment Approach to address social and linguistic disorders in the future. Finally, evaluating the mechanisms by which the cost-effective treatment ameliorates language delay and ASD symptomatology will improve our understanding of how the social environment can affect the developmental trajectories of Sibs-ASD. In a sample of 120 Sibs-ASD (12-15 mos), recruited at two sites (University of Washington, Seattle; Vanderbilt University, Nashville), this project will accomplish the following goals: (a) test the efficacy of a cost-effective treatment that shows promise for treatment of social and linguistic impairments in Sibs-ASD (ImPACT); (b) identify characteristics of the subgroup most benefiting from ImPACT; (c) evaluate whether improvements in pivotal skills produce anticipated effects on more distal spoken language skills and ASD symptomatology; (d) identify the active ingredients in ImPACT by which the treatment improves pivotal skills. Treatment efficacy goals will be addressed by stratifying Sibs-ASD on initial cumulative-risk status for communication disorder, including ASD, and then randomly assigning them to treatment or control groups. Treatment will be provided by parents under the supervision of research staff for 3 months. Immediate post-treatment evaluations will be followed by 3 month and 6 month follow-up assessments. Examiners and coders will be blind to treatment assignment. Intent to treat analyses will be conducted. State-of-the-art statistical methods will be used to identify the characteristics associated with response to ImPACT and to test whether pivotal skills and active ingredients of the treatment mediate treatment effects. The research team is experienced with the methods and has a productive collaborative history.